Many industrial applications require a shaft to pass from one area of a structure through a wall to another area of the structure. The shaft generally passes through an opening in the wall and moves relative to the position of the wall. For example, the shaft can translate, rotate, or move in some combination of translation and rotation (e.g., twisting, bending, or stretching) about an axis of the shaft through the surface. A spatial clearance generally exists between the shaft and the opening in the wall to facilitate this movement.
Some applications that involve moving shafts require fluid isolation between the separate areas of the structure so that a leak or contamination in one area of the structure does not migrate or flow to an adjacent area of the structure via the clearance between the shaft and the opening in the wall. For example, a propulsion shaft that extends along the length of the hull of a ship passes through several bulkheads that separate different compartments of the ship. Preventing a water leak in one compartment from advancing past a bulkhead into the next compartment along the shaft is critical in preventing the hull from filling with water and sinking the ship. Shaft seals are typically used to limit the flow of fluids from one compartment to the next during, for example, rotation of the shaft relative to the shaft opening in the ship structure.
An example of a conventional shaft seal designed to rotate as the shaft rotates during shaft operation is the ND-type shaft seal sold by Wartsila-Lips, Inc. of Poulsbo, Wash. The ND-type seal includes an o-ring positioned at a rubber molding-shaft interface, such that both the molding and the o-ring spin as the shaft spins. A pressure differential across the o-ring generated by an unequal amount of pressure on either side of a bulkhead causes the rubber molding to deform and press against the housing, which is positioned about the opening. Rotation of the molding is hindered when the molding presses against the housing. The stationary molding creates a seal against the housing and forces the o-ring into contact with the shaft. The o-ring also stops spinning and creates a fluid seal with respect to the shaft.
One drawback of the ND-type shaft seal is that contact between the sealing components and the shaft while the sealing components are dry leads to premature failure of the sealing components caused by the associated friction-induced wear on the o-ring and the rubber molding. Current shipbuilding specifications generally require a pressure differential between the opposing surfaces of the sealing components of about ⅓ psig (pounds per square inch gauge or about 234.1 kg/m2), or roughly 8-9 inches (about 203-229 mm) of water before the seal engages the shaft. The presence of water assists in creating a water-tight interface between the shaft and the sealing components (e.g., the molding and the o-ring). The ND-type seals have activated (e.g., contacted the shaft) in the presence of as little as 0.6-0.9 inches (about 15.2-22.9 mm) of water. The sealing components contact the shaft while the sealing components are relatively dry and lead to premature wear. Additionally, a pressure differential insufficient to generate a fluid-tight seal develops between opposing sealing components when the sealing components are activated in the presence of relatively small quantities of water. Where a fluid-tight seal does not develop, leakage rates associated with the shaft and sealing components can be in excess of shipbuilder specifications.
For example, shipbuilder specifications generally require self-activating bulkhead shaft seals with a maximum leakage rate of 0.5 U.S. pint/hour (about 0.065 ml/s). Self-activating shaft seals generally do not require human operation (e.g., adjustment of the seals) after installation with respect to a shaft and during a leak. For the DDG-type destroyer, the maximum leakage rate permitted under the shipbuilder specifications is 1 U.S. pint/minute (about 0.1314 ml/s). Activation of the seal in the presence of a relatively low pressure differential (e.g., premature activation caused by relatively low water levels discussed above) causes accelerated wear of the seal components. Accelerated wear of a seal leads to premature failure of the seal and noncompliance with shipbuilder specifications.
Hence, there is a need for self-activating shaft seals that do not prematurely activate in the presence of relatively small amounts of fluid. There also is a need for shaft seals designed to resist premature wear. There also is a need for shaft seals whose design can be scaled to effectively seal shafts of a variety of diameters. For example, there is a need for shaft seal designs for relatively small diameter shafts and relatively large diameter shafts, both of which can be found on destroyers or commercial ships, various industrial applications.